1. Field of the Invention
The present invention relates to a method and apparatus to preheat combustion air while cooling a hot process gas such as flue gas close to its dew point without any condensation. A heat transfer element in the form of multiple tubes, a serpentine coil, or metal conduit extends between two adjacent chambers through which the air and process or flue gas pass. Heat is transferred from the hot process gas to a liquid flowing in the coil and this heat is then in turn transferred from the liquid to heat the air.
2. Description of the Prior Art
The simplest form of air preheater involves a conduit or plate section in which there is a cross flow allowing hot flue gas to preheat the air being fed into combustion. Such a simple system is shown in FIG. 1. In this method of transfer essentially bare surface is utilized as both flue gas and air are at essentially atmospheric pressure and have relatively low heat transfer coefficients. This is in contrast to the heat transfer situation in boiler water economizers where there is an opportunity to use an enhanced or extended surface such as by finned tubes. On the low pressure side, of an economizer the fins provided extra surface to match the high heat transfer coefficients on the liquid side. In normal air preheating situations such surface enhancement is not practical without extended surface on both low pressure flue gas and low pressure inlet air sides. Some forms of extension of both sides of transfer surfaces have been achieved. However, the extension ratio, which is the ratio of surface exposed to the fluid as compared to the base plate or tube area, is limited and the apparatus is expensive. Through the use of an intermediate transfer liquid in the invention it is possible to use an economic low cost extended transfer surface on both the cooled and heated sides.
By analysis of FIG. 1 there is a theoretical limit to a simple cross flow condition as the air cannot reach a preheated temperature level in excess of the lowest flue gas temperature level. This limits the amount of air preheat and limits the efficiency.
A second form of air preheater with an intermediate liquid utilizes separate heat recovery surfaces in the flue gas and in the air ducts, as shown in FIG. 2. This systems avoids the limitations of FIG. 1, but it is necessary to balance the flowing heat capacity of the circulating liquid exactly to that of the flowing gases. In this system all the heat is first taken up in the liquid stream and is then transferred in total to the heated air. Thus at point A in FIG. 2 a cool liquid contacts a cooled flue gas with a positive likelihood of condensation and active corrosion conditions existing.
The Smith U.S. Pat. No. 3,623,549 discloses a heat exchange apparatus in which heat is absorbed in a liquid from a hot gas in one location and is transferred to relatively cool gas in a spaced apart second location. There are two or more separate heat transfer circuits between the hot gas and the cold gas. After the liquid has its temperature increased in the hot gas section, the liquid makes only one pass to the cool gas zone where it will give up its heat so that the liquid temperature decreases. Each conduit is completely independent of the other circuits, and each circuit is required to use a different heat transfer fluid. Furthermore, each time the heat transfer liquid passes from the hot zone to the cool zone a pump is required.
German Pat. No. 187,553 discloses a single heat transfer tube coiled back and forth between a cold gas zone on one side and a hot gas zone on the other side. There is a pump means with a cylinder of such capacity that each stroke displaces the heat transfer fluid in the tube from one side. On the return stroke, the fluid returns to its original position. This back and forth motion of the same fluid within the single tube does not permit the fluid to continuously flow in one direction completely through the apparatus.